1. Field of the Invention
The present invention relates to a fixing apparatus capable of preventing the excessive temperature rise of a heating roller, and an image forming apparatus having the fixing apparatus.
2. Description of Related Art
For image forming apparatuses such as copiers and printers, a heating-type fixing apparatus is widely used to fix a toner image transferred onto a recording sheet, on the recording sheet. The heating-type fixing apparatus is provided with a heating roller having heating means such as a heater and a pressure roller pressed against the heating roller. The recording sheet where a toner image has been transferred is passed between the heating roller and the pressure roller while being sandwiched therebetween, the toner on the recording sheet is fused, and pressurization is further performed, whereby the toner image is fixed on the recording sheet.
In such a fixing apparatus, it is necessary to accurately control the surface temperature of the heating roller in order to reliably fuse the toner on the recording sheet and prevent an adverse effect on the recording sheet. Therefore, conventionally, a plurality of thermistors are pressed against the surface of the heating roller, the temperatures of the center and edge of the surface of the heating roller are detected and the power supply to the heater is controlled, whereby the overall surface temperature of the heating roller is maintained uniform.
However, when the surface temperature of the heating roller is accurately measured by using the thermistors, it is necessary to press the thermistors against the surface of the heating roller with a predetermined pressure. For this reason, the thermistors are continuously pressed against the same parts of the heating rollers, so that the surface of the heating roller is deteriorated due to the friction between the thermistors and the surface of the heating roller and this degrades the fixing performance. In addition, since dirt on the surface of the heating roller adheres to the surfaces of the thermistors, the accurate temperature cannot be detected.
Therefore, to solve these problems, a fixing apparatus and an image forming apparatus are proposed in which the surface temperature of the heating roller is detected by an infrared sensor in a noncontact manner. For example, a fixing apparatus and an image forming apparatus are proposed in which even when the infrared emissivity of the heating roller differs according to the difference in color or material quality, the surface temperature of the heating roller can be accurately detected by correcting the temperature detected by the infrared sensor based on an emissivity-responsive signal corresponding to the infrared emissivity of the heating roller (see Japanese Laid-Open Patent Application No. 2000-227732).
Moreover, an image forming apparatus is proposed in which two areas having different infrared output characteristics are provided in a predetermined area of the heating roller and by detecting the temperatures of the two areas by an infrared sensor, temperature detection can be highly accurately performed even when the surface color of the heating roller is different (see Japanese Laid-Open Patent Application No. 2001-109316).
The noncontact-type temperature sensors described in Patent Document 1 and Patent Document 2 detect the surface temperature of the heating roller by detecting the infrared rays emitted from the surface of the heating roller, and has inside an infrared detecting thermistor and a temperature compensating thermistor. The infrared detecting thermistor detects the infrared rays emitted from the surface of the heating roller, and the output voltage thereof depends on the ambient temperature (that is, the temperature of the infrared detecting thermistor itself). To compensate for such temperature dependence, it is necessary to detect the temperature of the infrared detecting thermistor itself. Therefore, the temperature compensating thermistor is disposed in a position near the infrared detecting thermistor and not affected by the infrared rays emitted from the surface of the heating roller.
The temperature sensor is structured so that the absolute temperature of the surface of the heating roller can be grasped by detecting the voltages across the two thermistors disposed as described above, and converts the voltages across the two thermistors into digital values by an AD converter and outputs the digital values after the conversion to the CPU. The CPU obtains the surface temperature of the heating roller based on the inputted digital values and a predetermined table by executing a predetermined program, and controls the power supply to the heating roller.
However, there is a possibility that the software processing performed by the CPU cannot appropriately control the power supply to the heating roller when an anomaly occurs such as when the CPU cannot perform the predetermined processing because of an unexpected bug or the like and cannot respond within a predetermined time to cause a timeout or when the processing is stopped due to inability to continue the processing.